Coriolis flow meters can be used in a number of applications. In drilling, the Coriolis meter can measure volume flow rates and density of the drilling fluid. For example, in a managed pressure drilling (MPD) system, fluid flow is typically measured using a Coriolis flow meter to determine lost circulation, to detect fluid influxes or kicks, to measure mud density, to monitor fluid returns, etc.
As is known, the Coriolis flow meter can measure the mass flow rate of a medium flowing through piping. The medium flows through a flow tube inserted in line in the piping and is vibrated during operation so that the medium is subjected to Coriolis forces. From these forces, inlet and outlet portions of the flow tube tend to vibrate out of phase with respect to each other, and the magnitude of the phase differences provides a measure for deriving the mass flow rate. In particular, difference in phase shift between inlet/outlet determines the mass flow rate while the effects of fluid on tube natural frequency is used to determine fluid density.
Using Coriolis flow meters in a drilling system provides a number of advantages. The Coriolis flow meter is not restricted to measuring only one particular type of fluid, and the meter can measure slurries of gas and liquids without changes in properties (temperature, density, viscosity, and composition) affecting the meter's performance. Additionally, the Coriolis flow meter simply uses flow tubes and does not require mechanical components to be inserted in the harsh flow conditions of the drilling fluid.
An example of how a Coriolis flow meter is used in a drilling system is schematically illustrated in FIG. 1. The Coriolis flow meter 40 is part of a choke manifold 30, which contains piping, choke valves 32, and associated equipment. Fluid flow from the rotating control device (RCD) 12 of a well passes through the choke manifold 30 before passing on to other parts of a MPD system (not shown), such as a mud gas separator, rig trough, etc.
One particular example of a Coriolis flow meter is disclosed in U.S. Pat. No. 6,513,393, which is reproduced as FIG. 2. A mass flow sensor 40 of a Coriolis mass flow/density meter shown here has a single straight flow tube 41, which has an inlet side and an outlet side. Arrangements other than a single straight flow tube 41 are already known in the art. The inlet side of the flow tube 41 is provided with a first flange 48a, and the outlet side is provided with a second flange 48b so that the mass flow sensor 40 can be inserted in a pressure-tight manner in a pipe through which a medium flows during operation.
The mass flow sensor 40 further includes a support means 42 with a first end plate 44a fixed to the inlet side of the flow tube 41, a second end plate 44b fixed to the outlet side of the flow tube 41, and a support tube 43 inserted between the first and second end plates 44a-b. The end plates 44a-b are connected with the flow tube 41 and the support tube 43 in a rigid and pressure-tight manner, particularly in a vacuum-tight manner. The flow tube 41 is thus mounted in a lumen of the support tube 43 between the end plates 44a-b in a self-supporting manner so that it can be set into vibration.
A vibration exciter 50 is positioned within the support means 42 between the flow tube 41 and the support tube 43, preferably midway between the first and second end plates 44a-b. In operation, this vibration exciter 50 sets the flow tube 41 into vibration at a mechanical resonant frequency, which, in turn, offers a measure of the instantaneous density of the medium.
The vibration exciter 50 may be a solenoid assembly operated by a time-variable exciting current, thus setting the flow tube 41 into vibration, with the inlet side and the outlet side vibrating out of phase with respect to each other as the medium passes through the flow tube 41. Within the support means 42, a first measuring means 52a and a second measuring means 52b are positioned at a given distance from each other along the flow tube 41 for measuring the vibrations. The measuring means 52a-b are preferably located at equal distances from the middle of the flow tube 41 and provide first and second measurement signals that are representative of the vibrations.
Naturally, the mass flow sensor 40 has some form of protection, such as a protective housing or case. Such a protective case keeps the sensor 40 from detrimental effects, external damage and the like. Additionally, such a protective case can provide a sealed, pressure-tight environment that allows the sensor 40 to be used with hazardous materials and at significant pressures should the flow tube 41 fail due to fatigue or bursting.
As shown here, for example, the mass flow sensor 40 is protected from environmental influences by a sensor housing 45. Both the support means 42 and all electric leads connected to the mass flow sensor 40 are accommodated in the housing 45. A transition 46 on the sensor housing 45 has an electronics housing 60 affixed. Typically, to protect components of the meter 40, the housing 45 and support 42 may be fluid tight to prevent moisture and contaminants from entering, which could disrupt operation and cause corrosion and damage.
In the electronics housing 60, exciter electronics and evaluation electronics as well as other circuits used for the operation of the Coriolis mass flow/density meter are accommodated. These circuits can include electronics for supplying power fed from an external power source, and/or communication electronics for data transmission between the Coriolis mass flow/density meter and an external signal processing unit.
Having an understanding of how a Coriolis flow meter is used in a drilling system and what a conventional Coriolis flow meter includes, discussion now turns to some of the limitations encountered when using the conventional Coriolis flow meter in a drilling system.
Currently, the manifold 30 for a MPD system as in FIG. 1 may be rated for up to 10,000-psi pressure. However, even though the meter's pressure rating depends on its size and materials, the Coriolis meter 40 is typically limited to a rating of less than 3,000-psi, and usually about 1,500 to 2,855-psi. For this reason, the Coriolis meter 40 must be downstream of the chokes 32 due to this pressure limitation. Additionally, the Coriolis flow meter 40 may be installed with a bypass valve (not shown) and pressure sensor. If a pressure limit is exceeded, the bypass valve is actuated to bypass flow around the meter 40 so drilling can continue at rates that may exceed the capacity of the meter 40. As expected, this adds further complexity to the system.
One way to increase the pressure rating of the Coriolis flow meter 40 is to increase the wall thicknesses of the flow tube(s) used for the meter 40. The increased wall thickness leads to the expectation that the meter 40 can handle higher fluid pressures. However, increasing the wall thickness can reduce or eliminate the response capabilities of the meter 40. In other words, raising the pressure capability of the flow meter 40 by making the flow tube(s) more robust or thicker tends to compromise the functional accuracy of the flow meter 40. This can make the flow meter capable of high pressure operation functionally impractical for managed pressure drilling applications.
For high pressure applications, a turbine flow meter instead of the Coriolis flow meter 40 could be used to make the desired measurements, but the accuracy of the turbine flowmeter at measuring a full range of flow rates is inferior to a Coriolis flow meter. In fact, managed pressure drilling requires a high level of flow-measurement accuracy so that use of a turbine flowmeter is not acceptable.
Furthermore, because the pressure limitation of the Coriolis meter 40 requires it to be downstream of the chokes 32, any gas in the drilling fluid can come out of solution during the pressure drop experienced across the chokes 32. When this occurs, the Coriolis meter 40 may not function properly.
Keeping the gas in solution for the Coriolis flow meter 40 after the chokes 32 has been partially controlled by adding a valve (not shown), orifice, or the like downstream of the Coriolis flow meter. The downstream valve or the like can supply adequate back pressure to the meter 40, thereby keeping the gas in solution and allowing the meter 40 to read the fluid flow rate with improved accuracy. Although this arrangement may offer better pressure control to keep the gas in solution, positioning such a valve downstream of the meter 40 does not enable to the Coriolis flow meter 40 to operate at higher pressures, and such a valve downstream of the meter 40 adds complexity to the drilling system.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.